Ofioliti, 2011, 36 (2), 167-189 167 MANTLE ORIGIN OF THE ANTRONA SERPENTINITES (ANTRONA OPHIOLITE, PENNINE ALPS) AS INFERRED FROM MICROSTRUCTURAL, MICROCHEMICAL, AND NEUTRON DIFFRACTION QUANTITATIVE TEXTURE ANALYSIS Paola Tartarotti*, , Michele Zucali*,°, Matteo Panseri*, Sabrina Lissandrelli**, Silvia Capelli*** and Bachir Ouladdiaf*** * Dipartimento di Scienze della Terra “Ardito Desio”, Università degli Studi di Milano, Italy. ** Via G. Marconi 66, 28844 Villadossola, Italy. *** Institut Laue-Langevin, Grenoble, France. ° CNR-IDPA, Sezione di Milano, Italy. Corresponding author: e-mail: [email protected] Keywords: Antrona ophiolite, serpentinite, mantle peridotite, neutron diffraction, Quantitative texture analysis. Pennine Alps. ABSTRACT The Antrona ophiolite is located in the Italian side of Western Central Alps. In the Alpine nappe stack, it lies at low structural levels, being sandwiched between the overlying continental Monte Rosa Nappe (upper Penninic) and the underlying Camughera-Moncucco continental Unit (middle Penninic). The ophiolite sequence includes serpentinized ultramafites, metagabbros and mafic rocks covered by calcschists. The ultramafic portion of the Antrona ophiolite consists of serpentinized peridotites, with interbedded layers of various mafic/ultramafic rocks, and underlies the mafic rocks and metasediments. In spite of the Alpine tectonic and metamorphic evolution of the Antrona ophiolite and its heavy serpentinization, the ultramafic rocks preserve relict texture and miner- alogy that allow discussing the nature of their protoliths. Olivine-clinopyroxene-spinel-bearing serpentinites still retain relict porphyroclastic texture, com- monly attributed to mantle peridotites. This inference is supported by quantitative textural analysis of Lattice Preferred Orientation by neutron diffraction, per- formed for the first time in olivine crystals of Alpine ophiolites, suggesting T conditions > 800°C for the activation of slip systems. Mineral chemistry of fresh olivine, clinopyroxene and spinel also contribute to support the mantle nature of the serpentinite protolith. The Antrona ophiolite may thus be regarded as a fossilized fragment of oceanic lithosphere including mantle rocks, volcanics and deep-see sediments. Although their extention is smaller, the Antrona ophio- lite seems to be comparable with a coherent ophiolitic slice, such as Zermatt Saas ophiolite more than with a serpentinite mélange. INTRODUCTION which has been regarded as a slow-spreading realm, compa- rable to the modern Atlantic Ocean (e.g., Lagabrielle and The Antrona ophiolite represents remnants of the oceanic Cannat, 1990; Lagabrielle and Lemoine, 1997; Tartarotti et lithosphere of the Mesozoic Western Tethys, now inserted as al., 1998). The internal setting of the Antrona ophiolite is tectonic slices in the Penninic nappe pile of the western Cen- still not well constrained, probably due to its small extent tral Alps. The Alpine tectono-metamorphic evolution of this and pervasive metamorphic retrogression. Consequently, ophiolite is characterized by subduction-related blueschist this ophiolite is often neglected and poorly considered for prograde path, followed by high-pressure (eclogitic) meta- paleogeographic reconstruction of the Western Tethys. morphic peak, and subsequent retrograde exhumation domi- In this paper, we focused our study on the ultramafic nated by epidote-amphibolite to amphibolite facies condi- rocks of the Antrona ophiolite by performing original geo- tions (Colombi and Pfeifer, 1986; Pfeifer et al., 1989; Car- logical field mapping and sampling, then integrated with rupt and Schlup, 1998; Turco and Tartarotti, 2006). As some microstructural and microchemical analyses. No published ophiolites of the Western Alps (e.g., Zermatt-Saas Unit) the geological maps are available in literature, with the excep- Antrona ophiolite still retains well-preserved portions of the tion of the Geological Sheet Nb. 15 “Domodossola” at scale oceanic lithosphere. One of the main discussion recently de- of 1:100.000 (Carta Geologica d’Italia, 1959). The studied veloped on the Western Alps ophiolites regards their internal samples are poorly serpentinized olivine-rich rocks and tectonic setting, which is considered crucial for understading dunites which still retain mineralogical and textural relics the subduction dynamics. Namely, two main settings have that allow discussing and interpreting their primary charac- been recognized: one represents serpentinite mélanges con- ters and inferring the rock protolith. Microstructural and mi- sisting of tectonic ophiolitic blocks inserted in a serpentinite crochemical investigations were complemented by a quanti- matrix, and showing a wide range of P-T conditions (e.g., tative texture analysis (LPO) by neutron diffraction on se- Monviso, Voltri, Blake et al., 1995; Guillot et al., 2004; Fed- lected olivine-rich samples, in order to investigate the inter- erico et al., 2007); the other represents relatively coherent nal fabric of relict fresh olivine. The results allow discussing tectonic slices with km-scale length and thickness, and the active slip systems during the rock deformation and re- showing almost homogeneous burial and exhumation pat- lating them with a mantle origin. terns (e.g., Zermatt-Saas ophiolite, Angiboust et al., 2009; The aim of this paper was therefore to investigate the na- 2011; Monviso ophiolite, according to Lombardo et al., ture of the ultramafic rocks of the Antrona ophiolite, in or- 1978; Castelli and Lombardo, 2007; Angiboust et al., 2011). der to shed light on the internal setting of this ophiolite se- All the mentioned ophiolites have been interpreted as deriv- quence, and to improve a comparison with other ophiolites ing from subduction and exhumation of the Tethyan Ocean of the Alps. 168 GEOLOGICAL SETTING metasediments (tabular quartzites, dolostones, carnieuls, calcschists) as well as ophiolites of the underlying Antrona Regional geology Unit (Blumenthal, 1952; Bearth, 1957; Dal Piaz, 1966; Wet- The study area is located in the Pennine Alps, west of the zel, 1972). According to Dal Piaz’s studies, the Furgg Zone Lepontine dome and SE of the Simplon Line (western part exposed in the southern Monte Rosa sectors has a pre- of the Central Alps; Fig. 1) where huge crystalline massifs granitic protolith, is free of Mesozoic rocks, and is affected are mantled by tectonic slices of ophiolite (e.g., Escher et al. by eclogitic to greenschists facies Alpine metamorphism 1987; 1993). In the internal Pennine Domain, the Zermatt- (Dal Piaz, 1964; 1966). More recently, the Furgg Zone has Saas ophiolite wraps the upper Penninic Monte Rosa Nappe been regarded as the cover of the Monte Rosa basement and disappears below the middle Penninic Mischabel back- (Jaboyedoff et al., 1996; Escher et al., 1997; Steck et al., fold; the Antrona ophiolite lies on the footwall of the Monte 2001) or of the continental Portjengrat Unit (Keller and Rosa Nappe and in turn overlays the middle Penninic Ca- Schmid, 2001). Alternatively, the Furgg Zone has been con- mughera-Moncucco Nappe (Bearth, 1956; Laduron, 1976; sidered as the suture of the Valais basin (Froitzheim, 1997; Bigioggero et al., 1981; Keller et al., 2005a). The Monte 2001) or a tectonic mélange interposed between the conti- Rosa Nappe is a large NW-vergent recumbent anticline, nental Monte Rosa-Portjengrat Units and the Zermatt-Saas strongly refolded by S-vergent backfolds of supposed and Antrona ophiolites (Kramer, 2002). The Monte Rosa Oligocene age (Milnes et al., 1981; Escher et al., 1997; Nappe, the Furgg Zone, the Camughera Moncucco Unit, Steck et al., 1997). Together with the Gran Paradiso and the and the Zermatt-Saas and Antrona ophiolite have all experi- Dora Maira “massifs”, the Monte Rosa Nappe is part of the enced regional subduction-related high-pressure metamor- inner and upper continental nappe system of the Penninic phism during the Alpine orogeny (Dal Piaz et al., 1972; Zone in the Western Alps (Argand, 1911); it consists of pre- Compagnoni et al., 1977; Ernst and Dal Piaz, 1978; Colom- Alpine high grade paragneisses rich in mid- to coarse- bi and Pfeifer, 1986; Dal Piaz and Lombardo, 1986; Borghi grained pegmatites (Bearth, 1952; Gosso et al., 1979; Dal et al., 1996; Michard et al., 1996; Keller et al., 2005a; Piaz, 1993; Keller and Schmid, 2001; Keller et al., 2005b; 2005b; Turco and Tartarotti, 2006). The Zermatt-Saas ophi- see also review in Dal Piaz, 2001) and of a granitic complex olite subduction history is attested by the occurrence of of Late Carboniferous and Permian ages (Hunziker, 1970; lozenge-shaped pseudomorphs and other prograde relics fol- Frey et al., 1976). During the Alpine orogeny, the pre- lowed by climax eclogitic to UHP mineral assemblage granitic high grade rocks were reworked into garnet micas- (Ernst and Dal Piaz, 1978; Oberhansli, 1980; Barnicoat and chists and albitic schists, and the granitic bodies into foliat- Fry, 1986; Martin and Tartarotti, 1989; Reinecke, 1991; ed, schistose or mylonitic ortogneisses. The Antrona ophio- 1998; Bucher et al., 2005; Angiboust et al., 2009). The lite is separated from the Monte Rosa basement by the eclogite facies metamorphism in the Zermatt-Saas Unit oc- Furgg Zone (Argand’s “synclinal de Furggen”; Argand, curred during the Tertiary (Bowtell et al., 1994; Amato et 1911). The Furgg Zone (Fig. 1) is made of micaschists, al- al., 1999; Dal Piaz et al., 2001). Eclogite rocks in the bitic schists, and leucocratic gneisses (Permian-Carbonifer-